The invention relates to an apparatus for the pretreatment and subsequent conveying, plastification, or agglomeration of plastics.
The prior art reveals numerous similar apparatuses of varying design, comprising a receiver (receiving container) or cutter compactor for the comminution, heating, softening and treatment of a plastics material to be recycled, and also, attached thereto, a conveyor or extruder for the melting of the material thus prepared. The aim here is to obtain a final product of the highest possible quality, mostly in the form of pellets.
By way of example, EP 123 771 or EP 303 929 describe apparatuses with a receiver and, attached thereto, an extruder, where the plastics material introduced into the receiver is comminuted through rotation of the comminution and mixing implements and is fluidized, and is simultaneously heated by the energy introduced. A mixture with sufficiently good thermal homogeneity is thus formed. This mixture is discharged after an appropriate residence time from the receiver into the screw-based extruder, and is conveyed and, during this process, plastified or melted. The arrangement here has the screw-based extruder approximately at the level of the comminution implements. The softened plastics particles are thus actively forced or stuffed into the extruder by the mixing implements.
Most of these designs, which have been known for a long time, are unsatisfactory in respect of the quality of the treated plastics material obtained at the outgoing end of the screw, and/or in respect of the quantitative output of the screw. Studies have shown that the requirements placed upon the screw downstream of the container, mostly a plastifying screw, differ during the course of the operation, and that this is attributable to container residence times that are longer for some batches of the product to be processed than for other batches. The average residence time of the material in the container is calculated by dividing the weight of the charge in the container by the amount discharged from the screw per unit of time. However, this average residence time is—as mentioned—generally not valid for large portions of the material to be processed, but instead there are irregular substantial upward and downward deviations from this average value. These deviations may be attributable to differences in the nature of the batches of product introduced successively into the container, e.g. differences in the nature or thickness of the plastics material, e.g. foil residues, etc., or else uncontrollable events.
For material that is thermally and mechanically homogeneous, there is usually a quality improvement in the product obtained at the outgoing end of the screw when the flight depth of the metering zone of the screw is very large and the screw rotation rate is kept very small. However, if it is desirable to increase the quantitative output of the screw or to improve the performance for example of a shredder-extruder combination, the screw rotation rate must then be raised, and this means that the shear level is also raised. However, this causes the screw to subject the processed material to higher mechanical and thermal stress, and there is therefore the risk of damage to the molecular chains of the plastics material. Another disadvantage that can arise is greater wear of the screw and of its housing, in particular during the processing of recycling material, by virtue of the contaminants present in this material, e.g. abrasive particles, metal parts, etc., which cause severe wear of the metal parts as they slide across one another, in the screw or in its bearings.
However, an effect that occurs both with slow-running and deep-cut screws (large flight depth) and with fast-running screws is that, as previously mentioned, differences in quality of individual batches of material introduced to the screw, e.g. differences in flake size and/or differences in temperature of the plastics material, have a disadvantageous effect with regard to inhomogeneity of the plastics material obtained at the outgoing end of the screw. In order to compensate for this inhomogeneity, the temperature profile of the extruder is in practice raised, and this means that additional energy has to be introduced into the plastic, thus subjecting the plastics material to the thermal damage mentioned and increasing the amount of energy required. Another result here is that the viscosity of the plastics material obtained at the outgoing end of the extruder is reduced, and this makes the material more free-flowing, with concomitant difficulties in the further processing of this material.
It can be seen from this that the process parameters that are advantageous for obtaining material of good quality at the outgoing end of the screw are mutually contradictory.
In an initial attempt to solve this problem, the diameter of the cutter compactor was increased in relation to the diameter of the screw. This enlargement of the container in comparison with conventional sizes improved the mechanical and thermal homogeneity of the plastics material pretreated in the container. The reason for this was that the ratio by mass of the continuously added untreated “cold” portions of material to the amount of material present in the container and already to some extent treated was smaller than under the conditions that usually prevail, and that the average residence time of the plastics material in the container was substantially increased. This reduction of the ratio by mass had an advantageous effect on the thermal and mechanical homogeneity of the material entering the screw housing from the container, and with this had a direct advantageous effect on the quality of the plastified or agglomerated material at the end of the extruder screw or of the agglomerating screw, since the product initially introduced to the screw was at least approximately of identical mechanical and thermal homogeneity, and therefore the screw itself was not required to achieve this homogeneity. The theoretical residence time of the treated plastics material in the container was approximately constant. Furthermore, this type of system with enlarged container was less sensitive than the known systems in relation to the accuracy of input portions.
Systems of this type were therefore in principle capable of effective use, and advantageous. However, although systems using containers or cutter compactors with large diameters, e.g. of 1500 mm or more, and with relatively long residence times, have good functionality, and although the quality of the recylate is high, they are not ideal in terms of space required and of efficiency, or they emit a large amount of heat.
Another feature shared by these known apparatuses is that the direction of conveying or of rotation of the mixing and comminution implements, and therefore the direction in which the particles of material circulate in the receiver, and the direction of conveying of the conveyor, in particular of an extruder, are in essence identical or have the same sense. This arrangement, selected intentionally, was the result of the desire to maximize stuffing of the material into the screw, or to force-feed the screw. This concept of stuffing the particles into the conveying screw or extruder screw in the direction of conveying of the screw was also very obvious and was in line with the familiar thinking of the person skilled in the art, since it means that the particles do not have to reverse their direction of movement and there is therefore no need to exert any additional force for the change of direction. An objective here, and in further derivative developments, was always to maximize screw fill and to amplify this stuffing effect. By way of example, attempts have also been made to extend the intake region of the extruder in the manner of a cone or to curve the comminution implements in the shape of a sickle, so that these can act like a trowel in feeding the softened material into the screw. Displacement of the extruder, on the inflow side, from a radial position to a tangential position in relation to the container further amplified the stuffing effect, and increased the force with which the plastics material from the circulating implement was conveyed or forced into the extruder.
Apparatuses of this type are in principle capable of functioning, and they operate satisfactorily, although with recurring problems:
By way of example, an effect repeatedly observed with materials with low energy content, e.g. PET fibres or PET foils, or with materials which at a low temperature become sticky or soft, e.g. polylactic acid (PLA) is that when, intentionally, stuffing of the plastics material into the intake region of the extruder or conveyor, under pressure, is achieved by components moving in the same sense, this leads to premature melting of the material immediately after, or else in, the intake region of the extruder or of the screw. This firstly reduces the conveying effect of the screw, and secondly there can also be some reverse flow of this melt into the region of the cutter compactor or receiver, with the result that flakes that have not yet melted adhere to the melt, and in turn the melt thus cools and to some extent solidifies, with resultant formation of a clump or conglomerate made of to some extent solidified melt and of solid plastics particles. This causes blockage on the intake and caking of the mixing and comminution implements. A further consequence is reduction of the throughput or quantitative output of the conveyor or extruder, since adequate filling of the screw is no longer achieved. Another possibility here is that movement of the mixing and comminution implements is prevented. In such cases, the system normally has to be shut down and thoroughly cleaned.
Problems also occur with polymer materials which have already been heated in the cutter compactor up to the vicinity of their melting range. If overfilling of the intake region occurs here, the material melts and intake is impaired.
Problems are also encountered with fibrous materials that are mostly orientated and linear, with a certain amount of longitudinal elongation and low thickness or stiffness, for example plastics foils cut into strips. A main reason for this is that the elongate material is retained at the outflow end of the intake aperture of the screw, where one end of the strip protrudes into the receiver and the other end protrudes into the intake region. Since the mixing implements and the screw are moving in the same sense or exert the same conveying-direction component and pressure component on the material, both ends of the strip are subjected to tension and pressure in the same direction, and release of the strip becomes impossible. This in turn leads to accumulation of the material in the said region, to a narrowing of the cross section of the intake aperture, and to poorer intake performance and, as a further consequence, to reduced throughput. The increased feed pressure in this region can moreover cause melting, and this in turn causes the problems mentioned in the introduction. Another problem is efficient and non-aggressive introduction of the material into the screw while avoiding blockages, and achieving greater intensity of treatment of the material in the container.